The present invention relates generally to highly accurate and robust control of induction machines from inverter drives without the use of rotor position or speed sensors for feedback. The terms "sensorless", "speed-sensorless", "transducerless", "tachless", and "encoderless" are all used interchangeably to describe such control methods.
Methods of achieving direct field orientation (DFO) of induction machines are described in Jansen et al., "Observer-Based Direct Field Orientation: Analysis and Comparison of Alternative Methods," IEEE Transactions on Industry Applications, Vol. 30, No. 4, July/August 1994, pp. 945-953. DFO is based upon estimation of either the rotor or stator flux from the terminal voltage and current. For rotor flux DFO systems, cross coupling between the torque current and rotor flux can be nearly eliminated by orienting the rotor flux as the d-axis reference frame with a rotor flux observer, and the torque command can be calculated using the estimated rotor flux from the observer to adjust the commanded rotor flux. A closed-loop rotor flux observer estimates flux primarily in the stationary frame and includes two open-loop rotor flux observers which are referred to as current and voltage models. The current model uses measured stator current and rotor position to produce a flux estimate, while the voltage model relies on the measured stator voltage and current. Jansen et. al. further describes a motor drive system including an induction motor driven by a current regulated pulse width modulating (PWM) amplifier, a torque and flux regulator, a synchronous-to-stationary frame transformation block, current sensors, voltage sensors, a three-two phase transformation block, a flux observer, stationary-to-synchronous frame transformation blocks, and a velocity observer.
A method for flux estimation in induction machines is described in Jansen et al., U.S. Pat. No. 5,559,419, issued Sep. 24, 1996, wherein AC drive power is supplied to stator windings at a fundamental drive frequency which is at a level sufficient to provide magnetic saturation in the stator and at a signal frequency which is substantially higher than the drive frequency and wherein the response of the stator windings is measured to determine the variation of the response as a function of time during operation of the motor to determine the angular position and/or the speed of the magnetic flux vector. More specifically, a heterodyne demodulator mixes a signal which is a function of the high signal frequency with the response from the stator windings to provide a signal indicative of the rotational position of the magnetic flux vector. A drawback of this embodiment is that the injected signal is a balanced rotating AC signal, which produces an undesirable torque ripple at the signal frequency.
A means of integrating the flux estimation scheme described in U.S. Pat. No. 5,559,419, with a DFO scheme to obtain wide-speed-range sensorless control down to and including sustained zero frequency operation, was demonstrated via simulation in Jansen et. al, "Transducerless Field Orientation Concepts Employing Saturation-induced Saliencies in Induction Machines," IEEE Transactions on Industry Applications, Vol. 32, No. 6, November/December 1996.
Blaschke et al., "Sensorless Direct Field Orientation at Zero Flux Frequency," IEEE-IAS Annual Meeting, October 1996, describes operating AC machines in a saturated condition and superimposing an AC test current vector on a command stator current vector. By injecting an AC test current on the rotor flux axis, the system of Blaschke et al. assumes that the rotor flux axis is aligned with the saliency axis. In practice, the two axes are not always sufficiently aligned. For many motor designs, especially with closed rotor slots, the rotor flux axis and the saliency axis may shift by up to 90 degrees when operating from no load to full load. A system that does not take this shift into account may not always be stable and may not always produce the desired torque and control performance.
Unlike the embodiment in U.S. Pat. No. 5,559,419 which tracks a saturation-induced variation in the stator transient inductance, the embodiment of Blaschke et al. attempts to track a saturation-induced variation in the magnetizing inductance. With most induction machines and with high frequency signal injection (&gt;50 Hz), the effects of the variation in stator transient inductance due to saturation will greatly outweigh the effects of the variation in magnetizing inductance as seen by the injected AC signal from the stator terminals. Thus a scheme designed to track variations in stator transient inductance caused by saturation is more desirable as a general purpose means of achieving zero frequency tachless control.